While Vue Composition API RFC Reference site has many advanced use scenarios with the watch module, there is no examples on how to watch component props?
Neither is it mentioned in Vue Composition API RFC's main page or vuejs/composition-api in Github.
I've created a Codesandbox to elaborate this issue.
<template>
<div id="app">
<img width="25%" src="./assets/logo.png">
<br>
<p>Prop watch demo with select input using v-model:</p>
<PropWatchDemo :selected="testValue"/>
</div>
</template>
<script>
import { createComponent, onMounted, ref } from "#vue/composition-api";
import PropWatchDemo from "./components/PropWatchDemo.vue";
export default createComponent({
name: "App",
components: {
PropWatchDemo
},
setup: (props, context) => {
const testValue = ref("initial");
onMounted(() => {
setTimeout(() => {
console.log("Changing input prop value after 3s delay");
testValue.value = "changed";
// This value change does not trigger watchers?
}, 3000);
});
return {
testValue
};
}
});
</script>
<template>
<select v-model="selected">
<option value="null">null value</option>
<option value>Empty value</option>
</select>
</template>
<script>
import { createComponent, watch } from "#vue/composition-api";
export default createComponent({
name: "MyInput",
props: {
selected: {
type: [String, Number],
required: true
}
},
setup(props) {
console.log("Setup props:", props);
watch((first, second) => {
console.log("Watch function called with args:", first, second);
// First arg function registerCleanup, second is undefined
});
// watch(props, (first, second) => {
// console.log("Watch props function called with args:", first, second);
// // Logs error:
// // Failed watching path: "[object Object]" Watcher only accepts simple
// // dot-delimited paths. For full control, use a function instead.
// })
watch(props.selected, (first, second) => {
console.log(
"Watch props.selected function called with args:",
first,
second
);
// Both props are undefined so its just a bare callback func to be run
});
return {};
}
});
</script>
EDIT: Although my question and code example was initially with JavaScript, I'm actually using TypeScript. Tony Tom's first answer although working, lead to a type error. Which was solved by Michal Levý's answer. So I've tagged this question with typescript afterwards.
EDIT2: Here is my polished yet barebones version of the reactive wirings for this custom select component, on top of <b-form-select> from bootstrap-vue (otherwise agnostic-implementation but this underlying component does emit #input and #change events both, based on whether change was made programmatically or by user interaction).
<template>
<b-form-select
v-model="selected"
:options="{}"
#input="handleSelection('input', $event)"
#change="handleSelection('change', $event)"
/>
</template>
<script lang="ts">
import {
createComponent, SetupContext, Ref, ref, watch, computed,
} from '#vue/composition-api';
interface Props {
value?: string | number | boolean;
}
export default createComponent({
name: 'CustomSelect',
props: {
value: {
type: [String, Number, Boolean],
required: false, // Accepts null and undefined as well
},
},
setup(props: Props, context: SetupContext) {
// Create a Ref from prop, as two-way binding is allowed only with sync -modifier,
// with passing prop in parent and explicitly emitting update event on child:
// Ref: https://v2.vuejs.org/v2/guide/components-custom-events.html#sync-Modifier
// Ref: https://medium.com/#jithilmt/vue-js-2-two-way-data-binding-in-parent-and-child-components-1cd271c501ba
const selected: Ref<Props['value']> = ref(props.value);
const handleSelection = function emitUpdate(type: 'input' | 'change', value: Props['value']) {
// For sync -modifier where 'value' is the prop name
context.emit('update:value', value);
// For #input and/or #change event propagation
// #input emitted by the select component when value changed <programmatically>
// #change AND #input both emitted on <user interaction>
context.emit(type, value);
};
// Watch prop value change and assign to value 'selected' Ref
watch(() => props.value, (newValue: Props['value']) => {
selected.value = newValue;
});
return {
selected,
handleSelection,
};
},
});
</script>
If you take a look at watch typing here it's clear the first argument of watch can be array, function or Ref<T>
props passed to setup function is reactive object (made probably by readonly(reactive()), it's properties are getters. So what you doing is passing the value of the getter as the 1st argument of watch - string "initial" in this case. Because Vue 2 $watch API is used under the hood (and same function exists in Vue 3), you are effectively trying to watch non-existent property with name "initial" on your component instance.
Your callback is called only once and never again. Reason it is called at least once is because new watch API is behaving like current $watch with immediate option (UPDATE 03/03/2021 - this was later changed and in release version of Vue 3, watch is lazy same way as it was in Vue 2)
So by accident you doing the same thing Tony Tom suggested but with wrong value. In both cases it's not valid code if you are using TypeScript
You can do this instead:
watch(() => props.selected, (first, second) => {
console.log(
"Watch props.selected function called with args:",
first,
second
);
});
Here the 1st function is executed immediately by Vue to collect dependencies (to know what should trigger the callback) and 2nd function is the callback itself.
Other way would be to convert props object using toRefs so it's properties would be of type Ref<T> and you can pass them as a 1st argument of watch
Anyway, most of the time watching props is just not needed - simply use props.xxx directly in your template (or setup) and let the Vue do the rest
I just wanted to add some more details to the answer above. As Michal mentioned, the props coming is an object and is reactive as a whole. But, each key in the props object is not reactive on its own.
We need to adjust the watch signature for a value in the reactive object compared to a ref value
// watching value of a reactive object (watching a getter)
watch(() => props.selected, (selection, prevSelection) => {
/* ... */
})
// directly watching a ref
const selected = ref(props.selected)
watch(selected, (selection, prevSelection) => {
/* ... */
})
Just some more info even though it's not the mentioned case in the question:
If we want to watch on multiple properties, one can pass an array instead of a single reference
// Watching Multiple Sources
watch([ref1, ref2, ...], ([refVal1, refVal2, ...],[prevRef1, prevRef2, ...]) => {
/* ... */
})
This does not address the question of how to "watch" properties. But if you want to know how to make props responsive with Vue's Composition API, then read on. In most cases you shouldn't have to write a bunch of code to "watch" things (unless you're creating side effects after changes).
The secret is this: Component props IS reactive. As soon as you access a particular prop, it is NOT reactive. This process of dividing out or accessing a part of an object is referred to as "destructuring". In the new Composition API you need to get used to thinking about this all the time--it's a key part of the decision to use reactive() vs ref().
So what I'm suggesting (code below) is that you take the property you need and make it a ref if you want to preserve reactivity:
export default defineComponent({
name: 'MyAwesomestComponent',
props: {
title: {
type: String,
required: true,
},
todos: {
type: Array as PropType<Todo[]>,
default: () => [],
},
...
},
setup(props){ // this is important--pass the root props object in!!!
...
// Now I need a reactive reference to my "todos" array...
var todoRef = toRefs(props).todos
...
// I can pass todoRef anywhere, with reactivity intact--changes from parents will flow automatically.
// To access the "raw" value again:
todoRef.value
// Soon we'll have "unref" or "toRaw" or some official way to unwrap a ref object
// But for now you can just access the magical ".value" attribute
}
}
I sure hope the Vue wizards can figure out how to make this easier... but as far as I know this is the type of code we'll have to write with the Composition API.
Here is a link to the official documentation, where they caution you directly against destructuring props.
In my case I solved it using key
<MessageEdit :key="message" :message="message" />
Maybe on your case would look something like this
<PropWatchDemo :key="testValue" :selected="testValue"/>
But I don't have any idea of its pros and cons versus watch
Change your watch method like below.
watch("selected", (first, second) => {
console.log(
"Watch props.selected function called with args:",
first,second
);
// Both props are undefined so its just a bare callback func to be run
});
None of the options above worked for me but I think I found a simple way that seems to works very well to keep vue2 coding style in composition api
Simply create a ref alias to the prop like:
myPropAlias = ref(props.myProp)
and you do everything from the alias
works like a charm for me and minimal
I have a Vue.js application which loads a list of items, and each item is passed as a prop to a Vue component.
I figured out that by using mixins I can share common component properties, like computed,created, etc.
Now, I'm trying to sort the list of items and can't figure out how I would access each component's computed properties to apply sorting/filtering. How can I accomplish this?
Items
[{
price: 10,
qty: 2
}, {
price: 8,
qty: 3
}]
Mixin - ./Cost.js
export default {
computed: {
cost () {
return this.price * this.qty;
}
}
}
Component (which works as expected) - ./Product.vue
import Cost from './Cost.js'
export default {
name: 'product-item',
props: ['product'],
mixins: [Cost]
}
How would you access the computed properties, or restructure this setup?
List component
<template>
<div id="list">
<div v-for="product in sorted" :product="product">Cost: {{ cost }} </div>
</div>
</template>
<script>
import ProductItem from './Product.vue'
export default {
components: { ProductItem },
created: () {
this.items = [...] // as noted above
},
computed: {
sorted () {
return this.items.sort( (a,b) => b.cost - a.cost); // cost is not accessible!
}
}
}
</script>
Use vuex. Your vuex store will provide a getters object that can be wrapped into multiple components’ native computed objects, or accessed directly. Your code will be DRY, reactive, cached, and maintainable.
From my experience, once you need to go beyond child-parent data relationships, vuex, store, and shared state are the way to go. Once you get the hang of it, it is downright magical how your app evolves.
It is beyond scope of the question to show how to install vuex. Visit https://vuex.vuejs.org/guide/getters.html to see how getters are similar to computed properties, with the value of being shared between components. The official Vuex guide will also demonstrate how to initialize your Vue instance with the store.
Here are some snippets to show you the actors in the vuex system.
Store and State
// state definition (basically a shared reactive 'data' object that lives outside components)
state:{
message:'Hello'
}
// the store getters are declared as methods and accessed as properties (just like component/computed)
getters:{
message: state => return state.message
}
Accessing From Components
// component 1 wraps getter
computed:{
message(){
return this.$store.getters.message
}
}
// component 2 also wraps getter
computed:{
message(){
return this.$store.getters.message
}
}
// templates can also use getters directly
<div>{{$store.getters.message}}</div>
// If message was wrapped, you can simply use the computed property
<div>{{message}}</div>
Once you start using vuex, all sorts of other treasures start to emerge, such as the developer tools in Chrome, undo/redo support, simple refactoring of state, time-travel debugging, app persistence, etc. There are also shortcuts for adding multiple store getters into your computed properties.
As suggested by #Sphinx, you could use a ref to access the child component.
For example:
<template>
<div id="list">
<product-item v-for="product in sorted" :product="product" :ref="product"></product-item>
</div>
</template>
<script>
import ProductItem from './Product.vue'
export default {
components: { ProductItem },
data: () => ({
hidrated: false,
items: []
})
created() {
this.items = [...] // as noted above
},
mounted() {
this.hidrated = true
},
computed: {
sorted () {
if (!this.hidrated && !Object.keys(this.$refs).length) {
// handle initial state, before rendered
return this.items
}
return Object.values(this.$refs)[0]
.sort((a,b) => b.cost - a.cost)
.map(c => c.product)
}
}
}
</script>
This is assuming you have no other ref in your List Component.
You also have to check if the component is rendered first, here I use hidrated to flag when the component is mounted.
I have the following component with a slot:
<template>
<div>
<h2>{{ someProp }}</h2>
<slot></slot>
</div>
</template>
For some reasons, I have to manually instantiate this component. This is how I am doing it:
const Constr = Vue.extend(MyComponent);
const instance = new Constr({
propsData: { someProp: 'My Heading' }
}).$mount(body);
The problem is: I am not able to create slot contents programmatically. So far, I can create simple string based slot:
const Constr = Vue.extend(MyComponent);
const instance = new Constr({
propsData: { someProp: 'My Heading' }
});
// Creating simple slot
instance.$slots.default = ['Hello'];
instance.$mount(body);
The question is - how can I create $slots programmatically and pass it to the instance I am creating using new?
Note: I am not using a full build of Vue.js (runtime only). So I don't have a Vue.js compiler available to compile the template on the fly.
I looked into TypeScript definition files of Vue.js and I found an undocumented function on Vue component instance: $createElement(). My guess is, it is the same function that is passed to render(createElement) function of the component. So, I am able to solve it as:
const Constr = Vue.extend(MyComponent);
const instance = new Constr({
propsData: { someProp: 'My Heading' }
});
// Creating simple slot
const node = instance.$createElement('div', ['Hello']);
instance.$slots.default = [node];
instance.$mount(body);
But this is clearly undocumented and hence questionable approach. I will not mark it answered if there is some better approach available.
I think I have finally stumbled on a way to programmatically create a slot element. From what I can tell, the approach does not seem to work for functional components. I am not sure why.
If you are implementing your own render method for a component, you can programmatically create slots that you pass to child elements using the createElement method (or whatever you have aliased it to in the render method), and passing a data hash that includes { slot: NAME_OF_YOUR_SLOT } followed by the array of children within that slot.
For example:
Vue.config.productionTip = false
Vue.config.devtools = false;
Vue.component('parent', {
render (createElement) {
return createElement('child', [
createElement('h1', { slot: 'parent-slot' }, 'Parent-provided Named Slot'),
createElement('h2', { slot: 'default' }, 'Parent-provided Default Slot')
])
}
})
Vue.component('child', {
template: '<div><slot name="parent-slot" /><slot /></div>'
})
new Vue({
el: '#app',
template: '<parent />'
})
<script src="https://cdn.jsdelivr.net/npm/vue#2.5.17/dist/vue.js"></script>
<div id='app'>
</div>
(This doesn't really answer How to create Vue.js slot programatically?. But it does solve your problem.)
This trick is less hackish compared to using $createElement().
Basically, create a new component that register MyComponent as a local component.
const Constr = Vue.extend({
template: `
<MyComponent someProp="My Heading">
<div>slot here !!!</div>
</MyComponent>
`,
components: {
MyComponent: MyComponent
}
});
const instance = new Constr().$mount('#app');
Demo: https://jsfiddle.net/jacobgoh101/shrn26p1/
I just came across an answer to this in vue forum:
slots
The principle is: There is nothing like createElement('slot'..)
Instead there is a render function which provides the slotted innerHtml as function:
$scopedSlots.default()
Usage:
render: function (createElement) {
const self = this;
return createElement("div", this.$scopedSlots.default());
}
If you want to provide a default in case there is no content given for the slots, you need to code a disctinction yourself and render something else.
(The link above holds a more detailed example)
The function returns an array, therefore it can not be used as a root for the render function. It need to be wrapped into single container node like div in the example above.
I started https://laracasts.com/series/learning-vue-step-by-step series. I stopped on the lesson Vue, Laravel, and AJAX with this error:
vue.js:2574 [Vue warn]: Avoid mutating a prop directly since the value will be overwritten whenever the parent component re-renders. Instead, use a data or computed property based on the prop's value. Prop being mutated: "list" (found in component )
I have this code in main.js
Vue.component('task', {
template: '#task-template',
props: ['list'],
created() {
this.list = JSON.parse(this.list);
}
});
new Vue({
el: '.container'
})
I know that the problem is in created() when I overwrite the list prop, but I am a newbie in Vue, so I totally don't know how to fix it. Does anyone know how (and please explain why) to fix it?
This has to do with the fact that mutating a prop locally is considered an anti-pattern in Vue 2
What you should do now, in case you want to mutate a prop locally, is to declare a field in your data that uses the props value as its initial value and then mutate the copy:
Vue.component('task', {
template: '#task-template',
props: ['list'],
data: function () {
return {
mutableList: JSON.parse(this.list);
}
}
});
You can read more about this on Vue.js official guide
Note 1: Please note that you should not use the same name for your prop and data, i.e.:
data: function () { return { list: JSON.parse(this.list) } } // WRONG!!
Note 2: Since I feel there is some confusion regarding props and reactivity, I suggest you to have a look on this thread
The Vue pattern is props down and events up. It sounds simple, but is easy to forget when writing a custom component.
As of Vue 2.2.0 you can use v-model (with computed properties). I have found this combination creates a simple, clean, and consistent interface between components:
Any props passed to your component remains reactive (i.e., it's not cloned nor does it require a watch function to update a local copy when changes are detected).
Changes are automatically emitted to the parent.
Can be used with multiple levels of components.
A computed property permits the setter and getter to be separately defined. This allows the Task component to be rewritten as follows:
Vue.component('Task', {
template: '#task-template',
props: ['list'],
model: {
prop: 'list',
event: 'listchange'
},
computed: {
listLocal: {
get: function() {
return this.list
},
set: function(value) {
this.$emit('listchange', value)
}
}
}
})
The model property defines which prop is associated with v-model, and which event will be emitted on changes. You can then call this component from the parent as follows:
<Task v-model="parentList"></Task>
The listLocal computed property provides a simple getter and setter interface within the component (think of it like being a private variable). Within #task-template you can render listLocal and it will remain reactive (i.e., if parentList changes it will update the Task component). You can also mutate listLocal by calling the setter (e.g., this.listLocal = newList) and it will emit the change to the parent.
What's great about this pattern is that you can pass listLocal to a child component of Task (using v-model), and changes from the child component will propagate to the top level component.
For example, say we have a separate EditTask component for doing some type of modification to the task data. By using the same v-model and computed properties pattern we can pass listLocal to the component (using v-model):
<script type="text/x-template" id="task-template">
<div>
<EditTask v-model="listLocal"></EditTask>
</div>
</script>
If EditTask emits a change it will appropriately call set() on listLocal and thereby propagate the event to the top level. Similarly, the EditTask component could also call other child components (such as form elements) using v-model.
Vue just warns you: you change the prop in the component, but when parent component re-renders, "list" will be overwritten and you lose all your changes. So it is dangerous to do so.
Use computed property instead like this:
Vue.component('task', {
template: '#task-template',
props: ['list'],
computed: {
listJson: function(){
return JSON.parse(this.list);
}
}
});
If you're using Lodash, you can clone the prop before returning it. This pattern is helpful if you modify that prop on both the parent and child.
Let's say we have prop list on component grid.
In Parent Component
<grid :list.sync="list"></grid>
In Child Component
props: ['list'],
methods:{
doSomethingOnClick(entry){
let modifiedList = _.clone(this.list)
modifiedList = _.uniq(modifiedList) // Removes duplicates
this.$emit('update:list', modifiedList)
}
}
Props down, events up. That's Vue's Pattern. The point is that if you try to mutate props passing from a parent. It won't work and it just gets overwritten repeatedly by the parent component. Child component can only emit an event to notify parent component to do sth. If you don't like these restrict, you can use VUEX(actually this pattern will suck in complex components structure, you should use VUEX!)
You should not change the props's value in child component.
If you really need to change it you can use .sync.
Just like this
<your-component :list.sync="list"></your-component>
Vue.component('task', {
template: '#task-template',
props: ['list'],
created() {
this.$emit('update:list', JSON.parse(this.list))
}
});
new Vue({
el: '.container'
})
According to the VueJs 2.0, you should not mutate a prop inside the component. They are only mutated by their parents. Therefore, you should define variables in data with different names and keep them updated by watching actual props.
In case the list prop is changed by a parent, you can parse it and assign it to mutableList. Here is a complete solution.
Vue.component('task', {
template: ´<ul>
<li v-for="item in mutableList">
{{item.name}}
</li>
</ul>´,
props: ['list'],
data: function () {
return {
mutableList = JSON.parse(this.list);
}
},
watch:{
list: function(){
this.mutableList = JSON.parse(this.list);
}
}
});
It uses mutableList to render your template, thus you keep your list prop safe in the component.
The answer is simple, you should break the direct prop mutation by assigning the value to some local component variables(could be data property, computed with getters, setters, or watchers).
Here's a simple solution using the watcher.
<template>
<input
v-model="input"
#input="updateInput"
#change="updateInput"
/>
</template>
<script>
export default {
props: {
value: {
type: String,
default: '',
},
},
data() {
return {
input: '',
};
},
watch: {
value: {
handler(after) {
this.input = after;
},
immediate: true,
},
},
methods: {
updateInput() {
this.$emit('input', this.input);
},
},
};
</script>
It's what I use to create any data input components and it works just fine. Any new data sent(v-model(ed)) from parent will be watched by the value watcher and is assigned to the input variable and once the input is received, we can catch that action and emit input to parent suggesting that data is input from the form element.
do not change the props directly in components.if you need change it set a new property like this:
data() {
return {
listClone: this.list
}
}
And change the value of listClone.
I faced this issue as well. The warning gone after i use $on and $emit.
It's something like use $on and $emit recommended to sent data from child component to parent component.
one-way Data Flow,
according to https://v2.vuejs.org/v2/guide/components.html, the component follow one-Way
Data Flow,
All props form a one-way-down binding between the child property and the parent one, when the parent property updates, it will flow down to the child but not the other way around, this prevents child components from accidentally mutating the parent's, which can make your app's data flow harder to understand.
In addition, every time the parent component is updates all props
in the child components will be refreshed with the latest value. This means you should not attempt to mutate a prop inside a child component. If you do .vue will warn you in the
console.
There are usually two cases where it’s tempting to mutate a prop:
The prop is used to pass in an initial value; the child component wants to use it as a local data property afterwards.
The prop is passed in as a raw value that needs to be transformed.
The proper answer to these use cases are:
Define a local data property that uses the prop’s initial value as its initial value:
props: ['initialCounter'],
data: function () {
return { counter: this.initialCounter }
}
Define a computed property that is computed from the prop’s value:
props: ['size'],
computed: {
normalizedSize: function () {
return this.size.trim().toLowerCase()
}
}
If you want to mutate props - use object.
<component :model="global.price"></component>
component:
props: ['model'],
methods: {
changeValue: function() {
this.model.value = "new value";
}
}
I want to give this answer which helps avoid using a lot of code, watchers and computed properties. In some cases this can be a good solution:
Props are designed to provide one-way communication.
When you have a modal show/hide button with a prop the best solution to me is to emit an event:
<button #click="$emit('close')">Close Modal</button>
Then add listener to modal element:
<modal :show="show" #close="show = false"></modal>
(In this case the prop show is probably unnecessary because you can use an easy v-if="show" directly on the base-modal)
You need to add computed method like this
component.vue
props: ['list'],
computed: {
listJson: function(){
return JSON.parse(this.list);
}
}
Vue.component('task', {
template: '#task-template',
props: ['list'],
computed: {
middleData() {
return this.list
}
},
watch: {
list(newVal, oldVal) {
console.log(newVal)
this.newList = newVal
}
},
data() {
return {
newList: {}
}
}
});
new Vue({
el: '.container'
})
Maybe this will meet your needs.
Vue3 has a really good solution. Spent hours to reach there. But it worked really good.
On parent template
<user-name
v-model:first-name="firstName"
v-model:last-name="lastName"
></user-name>
The child component
app.component('user-name', {
props: {
firstName: String,
lastName: String
},
template: `
<input
type="text"
:value="firstName"
#input="$emit('update:firstName',
$event.target.value)">
<input
type="text"
:value="lastName"
#input="$emit('update:lastName',
$event.target.value)">
`
})
This was the only solution which did two way binding. I like that first two answers were addressing in good way to use SYNC and Emitting update events, and compute property getter setter, but that was heck of a Job to do and I did not like to work so hard.
Vue.js props are not to be mutated as this is considered an Anti-Pattern in Vue.
The approach you will need to take is creating a data property on your component that references the original prop property of list
props: ['list'],
data: () {
return {
parsedList: JSON.parse(this.list)
}
}
Now your list structure that is passed to the component is referenced and mutated via the data property of your component :-)
If you wish to do more than just parse your list property then make use of the Vue component' computed property.
This allow you to make more in depth mutations to your props.
props: ['list'],
computed: {
filteredJSONList: () => {
let parsedList = JSON.parse(this.list)
let filteredList = parsedList.filter(listItem => listItem.active)
console.log(filteredList)
return filteredList
}
}
The example above parses your list prop and filters it down to only active list-tems, logs it out for schnitts and giggles and returns it.
note: both data & computed properties are referenced in the template the same e.g
<pre>{{parsedList}}</pre>
<pre>{{filteredJSONList}}</pre>
It can be easy to think that a computed property (being a method) needs to be called... it doesn't
For when TypeScript is your preferred lang. of development
<template>
<span class="someClassName">
{{feesInLocale}}
</span>
</template>
#Prop({default: 0}) fees: any;
// computed are declared with get before a function
get feesInLocale() {
return this.fees;
}
and not
<template>
<span class="someClassName">
{{feesInLocale}}
</span>
</template>
#Prop() fees: any = 0;
get feesInLocale() {
return this.fees;
}
Assign the props to new variable.
data () {
return {
listClone: this.list
}
}
Adding to the best answer,
Vue.component('task', {
template: '#task-template',
props: ['list'],
data: function () {
return {
mutableList: JSON.parse(this.list);
}
}
});
Setting props by an array is meant for dev/prototyping, in production make sure to set prop types(https://v2.vuejs.org/v2/guide/components-props.html) and set a default value in case the prop has not been populated by the parent, as so.
Vue.component('task', {
template: '#task-template',
props: {
list: {
type: String,
default() {
return '{}'
}
}
},
data: function () {
return {
mutableList: JSON.parse(this.list);
}
}
});
This way you atleast get an empty object in mutableList instead of a JSON.parse error if it is undefined.
YES!, mutating attributes in vue2 is an anti-pattern. BUT...
Just break the rules by using other rules, and go forward!
What you need is to add .sync modifier to your component attribute in the parent scope.
<your-awesome-components :custom-attribute-as-prob.sync="value" />
Below is a snack bar component, when I give the snackbar variable directly into v-model like this if will work but in the console, it will give an error as
Avoid mutating a prop directly since the value will be overwritten whenever the parent component re-renders. Instead, use a data or computed property based on the prop's value.
<template>
<v-snackbar v-model="snackbar">
{{ text }}
</v-snackbar>
</template>
<script>
export default {
name: "loader",
props: {
snackbar: {type: Boolean, required: true},
text: {type: String, required: false, default: ""},
},
}
</script>
Correct Way to get rid of this mutation error is use watcher
<template>
<v-snackbar v-model="snackbarData">
{{ text }}
</v-snackbar>
</template>
<script>
/* eslint-disable */
export default {
name: "loader",
data: () => ({
snackbarData:false,
}),
props: {
snackbar: {type: Boolean, required: true},
text: {type: String, required: false, default: ""},
},
watch: {
snackbar: function(newVal, oldVal) {
this.snackbarData=!this.snackbarDatanewVal;
}
}
}
</script>
So in the main component where you will load this snack bar you can just do this code
<loader :snackbar="snackbarFlag" :text="snackText"></loader>
This Worked for me
Vue.js considers this an anti-pattern. For example, declaring and setting some props like
this.propsVal = 'new Props Value'
So to solve this issue you have to take in a value from the props to the data or the computed property of a Vue instance, like this:
props: ['propsVal'],
data: function() {
return {
propVal: this.propsVal
};
},
methods: {
...
}
This will definitely work.
In addition to the above, for others having the following issue:
"If the props value is not required and thus not always returned, the passed data would return undefined (instead of empty)". Which could mess <select> default value, I solved it by checking if the value is set in beforeMount() (and set it if not) as follows:
JS:
export default {
name: 'user_register',
data: () => ({
oldDobMonthMutated: this.oldDobMonth,
}),
props: [
'oldDobMonth',
'dobMonths', //Used for the select loop
],
beforeMount() {
if (!this.oldDobMonth) {
this.oldDobMonthMutated = '';
} else {
this.oldDobMonthMutated = this.oldDobMonth
}
}
}
Html:
<select v-model="oldDobMonthMutated" id="dob_months" name="dob_month">
<option selected="selected" disabled="disabled" hidden="hidden" value="">
Select Month
</option>
<option v-for="dobMonth in dobMonths"
:key="dobMonth.dob_month_slug"
:value="dobMonth.dob_month_slug">
{{ dobMonth.dob_month_name }}
</option>
</select>
I personally always suggest if you are in need to mutate the props, first pass them to computed property and return from there, thereafter one can mutate the props easily, even at that you can track the prop mutation , if those are being mutated from another component too or we can you watch also .
Because Vue props is one way data flow, This prevents child components from accidentally mutating the parent’s state.
From the official Vue document, we will find 2 ways to solve this problems
if child component want use props as local data, it is best to define a local data property.
props: ['list'],
data: function() {
return {
localList: JSON.parse(this.list);
}
}
The prop is passed in as a raw value that needs to be transformed. In this case, it’s best to define a computed property using the prop’s value:
props: ['list'],
computed: {
localList: function() {
return JSON.parse(this.list);
},
//eg: if you want to filter this list
validList: function() {
return this.list.filter(product => product.isValid === true)
}
//...whatever to transform the list
}
You should always avoid mutating props in vue, or any other framework. The approach you could take is copy it into another variable.
for example.
// instead of replacing the value of this.list use a different variable
this.new_data_variable = JSON.parse(this.list)
A potential solution to this is using global variables.
import { Vue } from "nuxt-property-decorator";
export const globalStore = new Vue({
data: {
list: [],
},
}
export function setupGlobalsStore() {
Vue.prototype.$globals = globalStore;
}
Then you would use:
$globals.list
Anywhere you need to mutate it or present it.